Number 265 (Story #1), April 5, 1996 by Phillip F. Schewe and Ben Stein|
HAS A SUPERSYMMETRIC PARTICLE BEEN PRODUCED? Just as all particles are believed to have antiparticle counterparts, so all known fermions (particles such as quarks and leptons with a half-integral intrinsic spin) are believed by some theorists to have hypothetical boson (particles with integral spins) counterparts, and vice versa. This "supersymmetry" hypothesis is part of an effort to unify all of the known particles and forces into a single theoretical framework. This work is complicated by the fact that many of the symmetries or unifying features that operated during the early universe are much different in the comparably colder universe of today. Thus many of the symmetries are "hidden" and forces such as electromagnetism and nuclear forces are very different in their effect on matter. Nevertheless, physicists in the lab can briefly recreate some of the conditions which pertained after the big bang. Just as accelerators were built to artificially create antiprotons in the 1950s, so scientists hope to produce some of the particles prophesied by the supersymmetry theory. In this regard, physicists have been talking for some months about a scattering event observed at the CDF detector at Fermilab's Tevatron. In this event, a high energy proton and antiproton collided head on; the outgoing debris included two electrons and two gamma rays (all with a high amount of energy flowing transverse to the beam axis), plus a great deal of transverse energy (about 53 GeV) which is "missing" or unaccounted for (still more missing energy may have eluded detection by narrowly hugging the beam axis). The CDF researchers have not yet published a paper based on this one event, but two groups of theorists have tried, Physical Review Letters, 10 June 1996, to provide some perspective. One (Savas Dimopoulos et al.; contact Michael Dine, UC Santa Cruz) reviews the likely decay signatures of supersymmetric particles and mentions the CDF event as one such candidate. The other paper (S. Ambrosanio et al.; contact Gordon Kane, University of Michigan, 313-764-4451) looks at the CDF event in detail, and suggests a scenario in which two "selectrons," supersymmetry counterparts of ordinary electrons, were created in the original collision. These exotic particles---with an inferred mass in the range 80-130 GeV---would then have decayed through a series of intermediate products, including the "photino," the supersymmetric partner of the photon. Kane and his colleagues argue that if their assessment is correct, there might be more as- yet-unrecognized supersymmetry candidate events in the present CDF data sample, and that such events may turn up in scattering experiments at CERN's LEP collider, running at 160 GeV this coming summer.